Store the last 20 headers reception time to a file

Summary:
And load it at startup. This is bootstrapping the RTT algorithm so the difficulty might only be over-estimated after a restart and miners won't mine rejected blocks.

This is opt-in because most nodes simply don't need the feature, it's an optimization that only impacts the miners.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit, PiRK

Reviewed By: #bitcoin_abc, roqqit, PiRK

Subscribers: PiRK, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16791